mikesmith wrote:
With deep holes to fill do people fill with several applications and
let the previous one dry out totally - or one "big" overfill and sand
down when thoroughly dry.

Yes, do it in layers (the instructions on the pack will advise this,
too). Otherwise if too much, the stuff will fall out of the hole before
it sets, and/or it will take an age for it to dry out properly anyway.

Also - I have a box I have had for at least three years - does it go
off (in the past its use by date sense) - I have noticed hairline
cracks in some of the filling I have done.

Certainly ordinary plaster goes off... it sets too quickly, which is
very likely to cause cracking. Polyfilla is chemically different but I
expect may behave similarly... personally I wouldn't use a 3-year-old pack.

David